1. Field of the Invention
The present invention relates to a method of inspecting the dimensions of a mask pattern of a photomask and a mask pattern dimensional inspection apparatus using this method.
2. Description of the Related Art
In recent years, various problems are becoming apparent in the photolithographic process used in the manufacture of semiconductor devices. As the dimensions of semiconductor devices have been scaled down, the demand has increased for fine patterning in the photolithographic process. The device design rules have already been scaled down to 45 nm. It is severely required that the accuracy of pattern dimensions be controlled below 5 nm. Furthermore, the mask pattern is formed into a very complex shape because of proximity-effect correction. For this reason, conventional one-dimensional dimensional uniformity inspection, such as mere measurements of the width of pattern lines and the diameter of holes, is insufficient and two-dimensional dimensional control is required. To meet the requirement, a method has been adopted which inspects whether or not a mask pattern has been finished to desired dimensions by converting the mask pattern into image data through SEM, extracting the pattern contour from the image data and conducting a lithographic simulation to determine whether a desired lithographic margin is obtained or not (see “Mask pattern quality assurance based on lithographic simulation with fine pixel SEM image” by M. Kariya et al, Proceedings of SPIE Vol. 5992).
The greatest merit of this method is that the finish of a mask pattern can be evaluated in a state very close to conditions in which the mask pattern is actually used to expose a wafer in a semiconductor manufacturing process. That is, the method can provide necessary and sufficient dimensional control and neither unnecessarily stringent dimensional control nor soft dimensional control.
However, in more stringent mask dimensional control nowadays, an insufficient point in terms of accuracy has emerged in the method as well. This is that the numerical aperture of wafer exposure apparatus has become large and the influence of sidewall shapes of mask pattern has become unnegligible. Specifically, the sidewall shapes of mask pattern vary according to the coverage of the mask pattern and positions in the mask plane. For this reason, trying to extract pattern contour data from SEM image data while neglecting sidewall shapes as in the conventional method has resulted in a case where the result of lithographic simulation using the contour data and the result of the actual exposure of wafer do not match.
It is generally known that the optical approach to obtain mask pattern contour data can provide accurate contour positions that better match the result of actual exposure of wafer than with the approach to obtain contour data from SEM image data. However, the optical approach is very laborious and hence the incorporation of it into the semiconductor device manufacturing process is not practical. In contrast, the approach to obtain pattern contour data from SEM image data can be easily implemented incorporated into the manufacturing process. However, when lithographic simulation is conducted using pattern contour data obtained by the conventional method, the result does not match the result of actual exposure of wafer. It is therefore difficult to accurately inspect the finish of a mask pattern.
The mask pattern dimensional inspection method of the invention involves previously determining a sidewall shape correction function representing the relationship of the difference between contour positions of two or more items of pattern contour position data of different thresholds obtained from an SEM image and optical pattern contour positions determined through an optical method, obtaining two or more items of pattern contour position data of different thresholds from SEM image data on which a lithographic simulation is to be conducted, determining pseudo-optical contour position data from the contour position difference and the sidewall shape correction function, conducting a lithographic simulation using the pseudo-optical contour position data, and evaluating whether or not a desired exposure margin is obtained on the basis of the result of the lithographic simulation.